The present invention relates to a process for the preparation of 3-hydroxyalkanals having 3 to 12 carbon atoms, in particular 3 or 4 carbon atoms, by hydration of the underlying 2-alkenals with water in a homogenous phase in the presence of an acid catalyst.
2-Alkenals of the general formula H.sub.2 C.dbd.CR--CHO in which R stands for hydrogen or alkyl, in particular acrolein and methacrolein, may be hydrated with water in the presence of acid catalysts (according to U.S. Pat. Nop. 2,434,110) to form the corresponding 3-hydroxyalkanals. 3-Hydroxypropionaldehyde (HPA) is obtainable from acrolein and may in turn be hydrogenated to produce 1,3-propanediol which is becoming increasingly important as monomer unit for polyesters and polyurethanes.
In the process according to U.S. Pat. No. 2,434,110, acids homogeneously dissolved in the reaction mixture, such as sulfuric acid, hydrochloric acid, phosphoric acid, oxalic acid, acid salts or acetic acid, are used as catalysts. Sulfuric acid was found to be the preferred catalyst in the prior art. The disadvantage of this process lies in the low yields and low selectivities.
Further processes have been developed to improve the selectivity of hydration of acrolein. Although suitable selectivities can be obtained by using carbon dioxide as catalyst (see British Patent 1,185,615), the long reaction time required considerably reduces the volume/time yield of this process.
Lastly, heterogeneous catalysts may be used, namely weakly acid ion exchangers containing carboxyl groups; see U.S. Pat. No. 3,536,763. In practice it was found that conventional ion exchangers containing carboxyl groups are limited in their activity and therefore required long reaction times. An improvement in volume/time yield combined with high selectivity was obtained by using ion exchangers containing phosphonic acid groups; see DE-OS 39 26 136.
As an alternative to using heterogeneous catalysts, there is a desire to find catalyst systems capable of operating in the homogeneous phase for hydration of the 2-alkenals with high selectivity as well as a satisfactory volume/time yield. Preferred catalyst systems should also be easily recoverable in the course of obtaining a secondary product from 3-hydroxyalkanals, such as in particular the alkane-1,3-diols obtainable from 3-hydroxyalkanals by hydrogenation. The reaction mixture from the hydration stage is normally only freed from unreacted 2-alkenal while 3-hydroxyalkanal is converted into a secondary product without being first isolated.